The present invention generally relates to methods and apparatus for processing semiconductor workpieces and, more particularly, to temperature control and monitoring during the processing of semiconductor workpieces.
During certain processing (e.g., plasma etching or deposition) of a semiconductor workpiece such as a semiconductor wafer of silicon, the workpiece is arranged on a susceptor disposed in a processing chamber. Electrostatic chucks are widely used to hold the semiconductor wafers in place on the susceptor during the processing using an electrostatic force produced by a DC bias voltage. The chuck may comprise an electrode arranged between two dielectric layers, wherein the electrode is connected to the DC bias voltage. FIG. 1 is a general block diagram of a conventional processing chamber which includes a processing chamber 10 defined by chamber walls 12. A semiconductor wafer W to be processed is arranged on a susceptor 14 and is held in place by an electrostatic chuck. The DC bias voltage for the electrostatic chuck is provided by a DC power supply 16. A vacuum port 18 is provided to evacuate processing chamber 10. Process gases for effecting a particular etching or deposition process are introduced into processing chamber 10. A magnetic field generator (not shown) (such as a solenoid or a permanent magnet) arranged outside the chamber and an RF power supply 20 are used to generate a plasma having ions and electrons which are incident on the wafer with a desired energy. The magnetic field generator is not necessary, but tools incorporate it to increase plasma density. Other wafer processing apparatuses are described in U.S. Pat. Nos. 5,567,267; 5,542,559; 5,462,603; 5,458,687; 5,382,311; and 5,290,381.
During processing, heat is generated by the energy of the ions and electrons incident on the wafer and the chemical reaction between the plasma and the wafer. With reference to FIG. 2, susceptor 14 is cooled using a cooling medium such as water which is circulated through a cooling medium passage 30 formed in susceptor 14. In order to control the temperature of the wafer W by transferring heat from the wafer to susceptor 14, helium or some other heat transfer gas is provided in a space 32 between the lower surface of the wafer W and the upper dielectric layer of the electrostatic chuck. The helium gas is provided to space 32 via a gas passage 34 formed in susceptor 14. By controlling the heat conductivity of the helium (and thus the amount of heat which is transferred to the cooled susceptor), the temperature of the wafer may be controlled. Since, in certain pressure ranges, the thermal conductivity of helium varies as a function of the pressure of the helium, controlling the pressure of the helium using a pressure controller 36 permits control of the thermal conductivity of the helium and therefore the amount of heat which is transferred from the heated wafer to the cooled susceptor.
It is often important to control the wafer temperature to be at a particular temperature or in a particular temperature range during processing. For example, if the temperature is too low, there is the possibility that water will be incorporated into the film being deposited on the wafer, which can adversely affect the operating characteristics and reliability of the completed device. On the other hand, a temperature which is too high can, for example, lead to undesirable melting of previously deposited metal wiring layers such as aluminum wiring layers and unwanted diffusion of impurities previously implanted in the wafer. When the helium is supplied to space 32 between wafer W and the electrostatic chuck, the helium generally leaks from the edges of the wafer as shown in FIG. 2 and the helium pressure is reduced at these edge portions. This reduction in pressure reduces the thermal conductivity of the helium at these edge portions. This reduced thermal conductivity reduces the amount of heat transferred to the susceptor 14 from the edge portions of the wafer as compared to the amount of heat transferred to the susceptor 14 from the central portions of the wafer. Thus, the temperature at these edge portions of the wafer is increased relative to the temperature at the central portions of the wafer. This non-uniform wafer temperature makes temperature control of the wafer difficult since different portions of the wafer will be at different temperatures.
The need to control the wafer temperature to be within a certain temperature range also makes it desirable to have a convenient mechanism for measuring wafer temperature. While wafer temperature may be easily measured with a thermocouple, this technique may be used only with monitor (test) wafers and not for actual production wafers. Wafer temperature may also be measured by detecting infrared emissions from the wafer. However, since silicon wafers are essentially transparent to infrared radiation, a detector for detecting infrared radiation will detect infrared radiation from the substrate, thereby resulting in an inaccurate temperature measurement.
A wafer processing apparatus includes a processing chamber, a chuck arranged in the processing chamber for supporting a wafer, and a pedestal that is spaced apart from the chuck. A first thermal transfer gas layer is provided between the chuck and the wafer and a second thermal transfer gas is provided in the space between the pedestal and the chuck. The pressure of the first thermal gas layer is controlled to be in a pressure range in which a thermal conductivity of the first thermal transfer gas is substantially constant with respect to changes in pressure and the pressure of the second thermal transfer gas layer is controlled so as to control an amount of heat transferred to the pedestal.
Since the pressure of the first thermal transfer gas layer between the wafer and the chuck is maintained in a range in which the thermal conductivity of the first thermal transfer gas does not vary significantly with respect to changes in the pressure of the first thermal transfer gas, leaks at the edge of the wafer do not significantly affect the temperature uniformity of the wafer. The temperature of the wafer and the chuck is controlled by the pressure of the second thermal transfer gas layer provided in a space between the chuck and the pedestal. The edges of this space are sealed with a thermal insulator and thus uniform thermal conductivity across the wafer may be provided. Since the first thermal transfer gas layer provides low thermal resistance, the temperature of the wafer is substantially the same as the temperature of the chuck. Thus, the temperature of the wafer may be determined using, for example, a thermocouple arranged on the chuck. This temperature information is supplied to a system control computer. Using this temperature information, the system control computer controls the pressure of the second thermal transfer gas layer to thereby control the temperature of the wafer and the chuck.
The present application also describes a wafer processing method and a control circuit for a wafer processing apparatus.
These and other features and aspects of the invention will be more clearly understood and better described if the following detailed description is read in conjunction with the appended drawings.